Certain solvents are used in industry to scrub gases. These solvents must be routinely cleaned and re-used. There are two general classes of solvents—chemical and physical. Chemical solvents react with impurities, while physical solvents remove impurities due to the solubility of the impurities in the solvents. Generally, pot distillation, ion exchange, and/or chemical treatments have been used to treat (reclaim) solvents after the solvent effectiveness is reduced. Solvents particularly needing reclamation include ethylene and/or propylene glycols, certain amines including alkanolamines, propropylene carbonate, NMP, as well as proprietary solvents such as di-alkyl ethylene and/or propylene glycols (of which Selexol, a mixture of the dimethyl ethers of polyethyleneglycol, is included). Certain solvents, such as Selexol, Sulfinol, and the like, are purely physical solvents, while certain chemical solvents such as ethanolamines react with impurities in the gases.
Similarly, there are three general classes of impurities which build up in the solvents. These include salts, solids, and heavy hydrocarbons.
After use, it may contain, in addition to water and glycols, specific contaminants according to origin and field of use. Glycol is often used to remove water from gas streams, and control hydrates in multiphase transmission pipelines. Further, large amounts of liquids containing glycol, especially ethylene glycol, are obtained in the manufacture of polyesters, especially polyester fibers; these liquids also contain, in addition to water, other impurities stemming from the process. Glycol streams, which include (poly)ethylene glycol, (poly)propylene glycol, (poly)ethylene/propylene glycols, and the like, typically contain water, solids, dissolved organic contaminants, and dissolved inorganic contaminants which are usually ionic. The glycol streams must be regenerated.
Large amounts of amine are used industrially. After use, it may contain, in addition to water and amine, specific contaminants according to origin and field of use. Amines, particularly alkanolamines, are used to remove acid gases from gas streams. Amine streams, which include alkanolamines such as monoethanolamine, diethanolamine, methyldiethanolamine, diisopropylamine, and aminoethoxyethanol, typically contain water, solids, dissolved organic contaminants, and dissolved inorganic contaminants which are usually ionic. The amine streams must be regenerated. Ion exchange or chemical precipitation are known methods of removing ionic components including salts, but these do not remove non-ionic organics. The constituents removed by ion exchange are typically characterized as amino acids and inorganic and organic salts which are formed in a petrochemical processes or are introduced in the gas treatment train process. When isolated as pure substances at room temperatures, these amino acids and salts are solids and are water soluble. These constituents are effectively removed by ion exchange processes at the expense of large volumes of easily treated waste brine water. The removal may be less than optimum and the introduction of water during treatment is often an inefficiency. Because these constituents are solids with high melting temperatures, vacuum distillation can also remove them by a very inefficient process of boiling of liquids. The distillation process is inefficient due to fouling of heat transfer surfaces, as well as additional solvent degradation due to the long exposure to heat and reactions between the solvent and impurities. Amine solvent may undergo degradation, whereby undesired degradation products are formed in the liquid phase. These degradation products, known as heat stable salts or heat stable amine salts, may accumulate in the circulating absorbent stream. For amine streams, the constituents not removed by ion exchange are chemical products of dehydration or reaction with carbon dioxide. This class of chemical substances can contain diamines, triamines, urea, esters and a myriad of other named and unknown chemical species. These species, when isolated at room temperature, can be solids; semi-solids; or viscous liquids and do not tend to exhibit significant ionic behavior when dissolved in water.
For glycol streams, particularly monoethylene glycol commonly employed to dehydrate natural gas as an initial step after gathering and prior to further processing, the process causes the glycol to absorb water and in the process become contaminated with naturally occurring and corrosion-generated salts. The glycol has to be dehydrated prior to being recycled in the natural gas production process. The glycol dehydration process is a type of distillation where the absorbed water is removed by heating and refined in a distillation column. The heat source in this process is a reboiler that applies the heat to a hot glycol at the bottom of this column contaminated with the salts and corrosion products resulting in a fouling condition. Use of straightforward distillation to remove water is known. However, distillation can result in hot temperatures for long durations, and various organics and inorganics in the glycol stream tend to react with one another and with the glycol to form undesirable byproducts. Cleaning a glycol stream with a Kettle Reboiler Reclaimer results in high solvent loss due to degradation. Frequent cleanout is required for reclaimer to work. This process is ineffective or in-operative in removing certain ionic components, particularly chlorides. The contaminant accumulates in reclaimer and in the circulation of the system until removed by a scheduled cleanout. Alternatively, kettle reboiling can be combined with a second ion exchange process to remove chlorides from systems with these reclaimers.
Vertically-oriented thin film evaporators have been used to evaporate certain selected solvents, where the thin film evaporators have plates orientated in a vertical direction, where a thin film of solvents flows over plates to speed up the process of evaporation of the solvent. The thin film may be mechanically agitated. Residual solvent carrying the impurities flows to the bottom of the plates where this material is disposed of or additionally reclaimed. Deficiencies include high solvent losses due to liquid (solvent) hold up in the vessel, bearing problems, thinning of the film, and slurry buildup caused by excess solvent in the bottoms is difficult to control. The contaminant removed is typically high in solvent content, typically having at least 15 wt % or more solvent.
Evaporation is a single-step in a distillation process, such as reboiling liquids in the bottom of a distillation column. Evaporation offers the possibility of removing all but a small fraction of both classes of undesirable constituents from contaminated amine solutions and salts from glycols in a single processing step. Evaporation in conventional evaporators has met with varying degrees of success due to the increased fouling of the heat transfer surfaces by the very contaminants the process was attempting to remove.
It is known to remove undesirable contaminants by vacuum distillation. In this process, water is typically removed first, and then the glycol itself is distilled off, leaving heavy organics and solids. This distillation process is not only effective on undesirable constituents that are not ionic in nature and cannot be removed by ion exchange, but also can remove the class of undesirable constituents that can be removed by ion exchange. The major drawbacks to the distillation process are poor heat transfer, degradation of heat transfer capability, the process necessity to waste significant quantities of the amines and glycols and for amines, their further degradation in the reclaimer. Further, distillation is typically a batch process, which is more labor intensive than steady state processes.
Forced Circulation Evaporator is also a process to clean solvents. Again, deficiencies include high solvent loss due to liquid hold up in the vessel and slurry build up in reclamation circulation. Further, the bottoms of such evaporator systems have high solvent content from 30 to 70 wt % depending on when the system is purged, resulting in a significant amount of solvent lost as the contaminant level is increased (from 3 to 20 wt %). This process is not considered commercially feasible.